Tsunamis: How Earthquakes Cause Them and the Warning Signs

By Quake Tracker Live Editorial · Published November 9, 2026

A tsunami is a series of ocean waves caused by a large, sudden movement of water, and earthquakes are the most common trigger. When a big earthquake lifts or drops a large patch of seafloor, the water above it is displaced and spreads outward as waves. If you are near the coast and feel strong or long shaking, see the ocean behave strangely, or hear a loud roar from the sea, move to high ground without waiting for an official message.

How an earthquake makes a tsunami

Most tsunamis start at subduction zones, where one tectonic plate slides beneath another. The plates can stay stuck together for centuries while stress builds. When the stuck section finally slips, the seafloor above it can rise or fall by several meters over an area as large as a small country. The sea surface above takes on the same shape, and gravity pulls it back toward level. That motion sets off waves that travel away in all directions.

NOAA's Tsunami Warning Centers describe a tsunami as the result of any large, sudden displacement of the ocean. Large undersea or near-shore earthquakes are the most common cause, but landslides, volcanic activity, certain weather events and even near-Earth objects can also generate tsunamis. In the Global Historical Tsunami Database, about 88% of tsunamis were caused by earthquakes or by landslides triggered by earthquakes.

Unlike wind waves, which only stir the surface, a tsunami moves the entire column of water from the surface to the seafloor. That is why it carries so much energy and why it can keep flooding for many minutes rather than breaking once and retreating.

Which earthquakes are most likely to cause tsunamis?

Not every earthquake near the ocean produces a tsunami. A few ingredients make one more likely:

• Location under or near the sea. The rupture has to move the seafloor, so inland earthquakes do not generate tsunamis. Magnitude is not the only factor; where the fault is matters too.
• Large magnitude. Tsunami-generating earthquakes are typically large. Our magnitude guide explains how much more energy each step on the scale releases.
• Shallow depth. A shallow rupture is more likely to disturb the seafloor than a deep one. See earthquake depth explained.
• Vertical movement. Faults that push the seafloor up or down, as subduction-zone thrust faults do, move far more water than faults whose sides slide past each other horizontally.

The largest earthquakes on record are almost all subduction-zone events, which is why so many of them are remembered for their tsunamis. Browse them in our guide to the largest earthquakes in the world. One example is the 1700 Cascadia earthquake, estimated at magnitude 9.0, whose tsunami struck the Pacific Northwest coast and crossed the ocean to Japan.

How fast and how high are tsunami waves?

NOAA notes that in the deep ocean a tsunami can travel faster than 500 miles per hour, about as fast as a jet plane, and the waves are often barely noticeable, rarely more than about three feet high. The speed depends on water depth: the deeper the water, the faster the wave. As a tsunami reaches shallow water near the coast it slows to roughly 20 to 30 miles per hour, and the water piles up into taller waves or surges.

Travel times show why distance matters. NOAA gives about five hours from the Aleutian Islands to Hawaii and about 8.5 hours from Portugal to North Carolina. A tsunami generated by an earthquake close to a coast, called a local tsunami, can reach it in less than an hour, sometimes in minutes.

Most tsunamis measure under 10 feet at the coast, but the biggest exceed 100 feet near their source, and a large one can flood low-lying land more than a mile inland.

Natural warning signs

Official alerts can be too slow when the source is close. NOAA lists three natural signs, and any one of them can mean a tsunami is on its way:

1. A strong or long earthquake. If shaking is hard to stand in or lasts a long time and you are near the coast, treat it as the warning.
2. A loud roar from the ocean. Some people describe it like a train or a jet.
3. Unusual ocean behavior. This can be a fast-rising flood or a wall of water, or the opposite: the sea suddenly drawing back and exposing the seafloor.

Do not go to the shore to watch a receding sea. The water that leaves comes back, and a tsunami usually arrives as a series of surges over hours, so the first wave may not be the largest.

Official alerts: watch, advisory and warning

In the United States, the National Tsunami Warning Center and the Pacific Tsunami Warning Center issue messages at four levels. They are not the same as the site you are reading now, and Quake Tracker Live is not an official alert system. Always follow your local emergency officials.

What to do if you are near the coast

• Move right away. If you feel strong or long shaking, or see another natural sign, go to high ground or inland. Do not wait for a siren or text.
• Walk if you can. Roads fill with traffic during evacuations. Know the route ahead of time.
• Go as high and as far as practical. Follow signs marking tsunami evacuation zones and routes, which many coastal communities post.
• Stay away until officials say it is safe. Waves can keep coming for hours.
• Drop, Cover and Hold On first if you are still being shaken. Then move. Our guide on what to do during an earthquake covers this.

Add a tsunami plan to your household preparations if you live, work or vacation near the coast. The earthquake preparedness checklist covers supplies, and your local emergency management office can show you the evacuation zone map for your address.

Where tsunami risk is highest in the U.S.

Alaska, Hawaii and the West Coast face the greatest tsunami exposure because they sit next to subduction zones and active Pacific faults. Check recent seismic activity for Alaska, Hawaii, Washington, Oregon and California, or see the biggest recent earthquakes worldwide. Internationally, countries such as Japan and Indonesia have long records of destructive tsunamis.

Two events that changed tsunami warning

The December 26, 2004 earthquake off northern Sumatra, Indonesia, produced a tsunami that killed about 230,000 people around the Indian Ocean, with waves reaching up to 167 feet on Sumatra's northern coast. The March 11, 2011 earthquake off Honshu, Japan, caused waves up to 128 feet along the Japanese coast and a death toll of 18,457, most from the tsunami. Both events led to major investments in detection buoys, warning centers and public education.

Frequently asked questions

Sources and further reading

• NOAA Tsunami Warning Centers: Tsunami FAQ
• NOAA National Tsunami Warning Program
• National Weather Service: Tsunami safety

This guide is general information, not professional or emergency advice. Quake Tracker Live is not an official warning system; follow instructions from local authorities.